Modulation of AC/AC MMC

ABSTRACT

A method of decoupled modulation of a direct AC/AC MMC between a first AC network L having a first waveform and a second AC network R having a second waveform includes performing first and second modulations based on respective reference signals of the first and second AC networks to generate, for each phase leg, first and second integer command signals corresponding to first and second combinations of cell states in the branches of the phase leg needed for generating the first and second waveforms. The method also includes, based on the first and second integer command signals, mapping to each branch a number of cell states to be used for concurrently generating both the first and second waveforms.

TECHNICAL FIELD

The present disclosure relates to modulation of direct alternatingcurrent (AC) to AC Modular Multilevel Converter (MMC).

BACKGROUND

An MMC is a power converter comprising series-connected cells (alsoknown as modules or submodules), forming what is called a converterbranch (also known as arm). These branches can be configured in severalmanners leading to dedicated converter topologies. According to whetherthis branch needs to provide only positive or also negative voltages,the cell can be implemented by means of a half-bridge or a full-bridge(also called bipolar or H-bridge) cell, respectively. Athree-to-single-phase direct AC/AC MMC structure in double-starconfiguration may be used for interconnection of a three-phase utilitygrid, e.g., 50 Hz, with e.g. a single-phase railway supply, e.g., at50/3 (synchronous) or 16.7 Hz (asynchronous).

SUMMARY

The present invention relates especially to a modulation problem ofmedium voltage direct AC/AC MMC, e.g. three-to-single-phase for railwayapplications. In this case and when a modulator is used on the level ofeach converter branch (modulation references, e.g. voltage referencesand/or flux references, are used for each branch during modulation), aninherent harmonic coupling is observed between the two AC networks (e.g.a three-phase network and a single-phase network) according to theirfrequency ratio mainly because of the lack of symmetry in the resultedmodulation patterns. This effect is more visible when the converter isoperated with low cell numbers and low switching frequencies. Moreover,more sophisticated modulators, such as optimized pulse patterns (OPPs),cannot be directly designed and applied on a branch level. Forsynchronous utility and railway grid operation, one would have to designa pattern featuring two different frequencies, which would be a quitecomplex procedure. The complexity of the problem becomes unrealisticallyhigh in asynchronous network operation, where one would have toadditionally account for all possible phase-shifts between the twonetworks.

A solution to the aforementioned problems is proposed based on adecoupled modulation concept. More specifically, each converter side,e.g. three- and single-phase, are modulated independently. Themodulation results are then mapped into branch level command signalsusing a mapping function and are then fed to a capacitor voltagebalancing algorithm, e.g., a sorting and selection algorithm for finaldetermination of each cell switching signal. According to the chosenmodulation method for each side, a choice for circulating currentcontrol implementation may be also used for more stable systemoperation.

One of the possible combined control/modulation method is ModelPredictive Pulse Pattern Control (MP3C), which is a fast closed-loopcontroller of OPPs. MP3C is based on the principle of Model PredictiveControl (MPC) and uses the so-called Receding Horizon Policy. MP3Callows the use of general OPPs with discontinuities in the switchingangles when changing the modulation index.

According to an aspect of the present invention, there is provided amethod of decoupled modulation of a direct AC/AC MMC between a first ACnetwork having a first waveform and a second AC network having a secondwaveform, the MMC having a double-star topology with a plurality ofphase legs, each phase leg having a first branch and a second branch,each of the first and second branches comprising a plurality of seriesconnected bipolar cells. The method comprises performing a firstmodulation based on a reference signal of the first AC network,independently of a reference signal of the second AC network, togenerate, for each phase leg, a first integer command signalcorresponding to a first combination of cells cell states in the firstand second branches of the phase leg needed for generating the firstwaveform. The method also comprises performing a second modulation basedon the reference signal of the second AC network, independently of thereference signal of the first AC network, to generate, for each phaseleg, a second integer command signal corresponding to a secondcombination of cell states in the first and second branches of the phaseleg needed for generating the second waveform. The method alsocomprises, based on the first and second integer command signals,mapping to each branch a number of cell states to be used forconcurrently generating both the first and second waveforms, generatingbranch-level command signals to a capacitor voltage balancing algorithm.The method also comprises, based on the mapping and the balancingalgorithm, sending firing signals to the plurality of cells of eachbranch. The method may e.g. be performed in/by a controller of the MMC.

According to another aspect of the present invention, there is provideda computer program product comprising computer-executable components forcausing a controller of an MMC to perform an embodiment of the method ofthe present disclosure when the computer-executable components are runon processor circuitry comprised in the controller.

According to another aspect of the present invention, there is provideda direct AC/AC MMC configured to be connected between a first AC networkhaving a first waveform and a second AC network having a secondwaveform, the MMC having a double-star topology with a plurality ofphase legs, each phase leg having a first branch and a second branch,each of the first and second branches comprising a plurality of seriesconnected bipolar cells. The MMC further comprises a controllercomprising processing circuitry, and storage storing instructionsexecutable by said processor circuitry whereby said controller isoperative to perform a first modulation based on a reference signal ofthe first AC network, independently of a reference signal of the secondAC network, to generate, for each phase leg, a first integer commandsignal corresponding to a first combination of cell states in the firstand second branches of the phase leg needed for generating the firstwaveform. The controller is also operative to perform a secondmodulation based on the reference signal of the second AC network,independently of the reference signal of the first AC network, togenerate, for each phase leg, a second integer command signalcorresponding to a second combination of cell states in the first andsecond branches of the phase leg needed for generating the secondwaveform. The controller is also operative to, based on the first andsecond integer command signals, map to each branch a number of cellstates to be used for concurrently generating both the first and secondwaveforms, generating branch-level command signals to a capacitorvoltage balancing algorithm. The controller is also operative to, basedon the mapping and the balancing algorithm, send firing signals to theplurality of cells of each branch.

It is to be noted that any feature of any of the aspects may be appliedto any other aspect, wherever appropriate. Likewise, any advantage ofany of the aspects may apply to any of the other aspects. Otherobjectives, features and advantages of the enclosed embodiments will beapparent from the following detailed disclosure, from the attacheddependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, step, etc.” are to be interpreted openly asreferring to at least one instance of the element, apparatus, component,means, step, etc., unless explicitly stated otherwise. The steps of anymethod disclosed herein do not have to be performed in the exact orderdisclosed, unless explicitly stated. The use of “first”, “second” etc.for different features/components of the present disclosure are onlyintended to distinguish the features/components from other similarfeatures/components and not to impart any order or hierarchy to thefeatures/components.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic circuit diagram of an embodiment of athree-phase-to-single-phase AC/AC MMC in accordance with the presentinvention.

FIG. 2 is a schematic circuit diagram of an embodiment of a bipolar cellof an MMC, in accordance with the present invention.

FIG. 3 is a schematic functional diagram of an embodiment of modulationand mapping performed in a controller of an MMC, in accordance with thepresent invention.

FIG. 4 is a schematic functional diagram of another embodiment ofmodulation and mapping performed in a controller of an MMC, inaccordance with the present invention.

FIG. 5 is a schematic functional diagram of another embodiment ofmodulation and mapping performed in a controller of an MMC, inaccordance with the present invention.

FIG. 6 is a schematic functional diagram of another embodiment ofmodulation and mapping performed in a controller of an MMC, inaccordance with the present invention.

FIG. 7 is a schematic functional diagram of another embodiment ofmodulation and mapping performed in a controller of an MMC, inaccordance with the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with referenceto the accompanying drawings, in which certain embodiments are shown.However, other embodiments in many different forms are possible withinthe scope of the present disclosure. Rather, the following embodimentsare provided by way of example so that this disclosure will be thoroughand complete, and will fully convey the scope of the disclosure to thoseskilled in the art. Like numbers refer to like elements throughout thedescription.

FIG. 1 is a schematic illustration of an MMC 1 in direct double-starconfiguration between a first AC network L, which is a three-phasenetwork having currents iL1, iL2 and iL3, and the, and a second ACnetwork R, which is a single-phase network having the current iR and thevoltage uR. The first AC network may e.g. be a national powerdistribution network which may have a utility frequency (powerline/mains frequency) of for instance 50 or 60 Hz. The second AC networkR may e.g. be for a railway electrification system, and may have autility frequency of for instance 25 Hz, 50/3 Hz (synchronous) or 16.70Hz (asynchronous).

The MMC 1 comprises a plurality of phase legs 11, here three (one perphase of the first AC network L), where each phase leg comprises a first(upper) branch (arm) 12 a and a second (lower) branch 12 b. Each branch12 comprises a plurality of series connected converter cells 13. In thefigure, the currents and voltages relating to the first branches 12 aare indexed “a” while the currents and voltages relating to the secondbranches 12 b are indexed “b”.

Typically, each branch 12 comprises the same number of cells 13.Embodiments of the present invention may be particularly useful for arelatively small number of cells per branch, why the number of seriesconnected cells in each branch may be less than 20, e.g. less than 15 orless than 10.

The MMC 1 also comprises a controller 14 which is schematically shown inFIG. 1. The controller may be a control system comprising a central unitand/or distributed units associated with respective legs 11 or branches12. The controller 14 may be configured, e.g. by means of computerprogramming, to perform embodiments of the method of the presentdisclosure.

The MMC 1 may be a medium voltage converter, e.g. having a voltagerating of less than 30 kV (thus being configured for an operatingvoltage of less than 30 kV) and/or a power rating within the range of 10to 60 MW, e.g. 15 to 30 MW (thus being configured for an operating powerwithin the range of 10 to 60 MW, e.g. 15 to 30 MW).

FIG. 2 illustrates an example of a bipolar cell 13. The cell comprisesan energy storing device 5, here in the form of a capacitor. The energystoring device 5 may comprise a capacitor arrangement with any number ofcapacitors in series and/or parallel connection with each other. Thecell 13 also comprises four semiconductor switches S, forming thefull-bridge (H-bridge) topology in the cell. Any number of semiconductorswitches may be used as long as the cell is still bipolar, and the cellwith four switches shown in the figure is only an example. Thesemiconductor switches of the bipolar cell are conventionally named inthe figure as S1 switch, S2 switch, S3 switch and S4 switch. When theswitches S1 and S4 are closed and S2 and S3 are open, the cell is in a+1 state in which a positive voltage will be applied. By opening S1 andS4 switches and closing S2 and S3 switches, this voltage is reversedwhereby the cell is in a −1 state and a negative voltage will beapplied. Each of the S switches may comprise e.g. an insulated-gatebipolar transistor (IGBT) or a gate commutated thyristor GCT (in whichcase a snubber circuit may also be needed), for instance an integratedgate commutated thyristor (IGCT), a reverse-conducting IGCT (RC-IGCT) ora bi-mode GCT (BGCT), possibly in combination with an antiparallelone-direction conducting/blocking component such as a diode. In theexample of FIG. 2, each S switch comprises an IGCT and antiparalleldiode.

Embodiments of the present invention may be especially advantageous forcells 13 having a relatively low switching frequency, e.g. of at most150 Hz, e.g. at most 100 Hz or 50 Hz.

Embodiments of the present invention relates to the modulation problemof medium voltage three-to-single-phase direct AC/AC MMC, e.g. forrailway applications. When a modulator is used on the level of eachconverter branch in accordance with prior art, a harmonic coupling isobserved between the two networks L and R according to their frequencyratio. This effect is more visible when the converter is operated withlow cell numbers and low switching frequencies. Moreover, moresophisticated modulators, such as optimized pulse patterns (OPPs),cannot be directly designed and applied on a branch level. Forsynchronous utility or railway grid operation, one would have to designa pattern featuring two different frequencies, which would be a quitecomplex procedure. The complexity of the problem becomes unrealisticallyhigh in asynchronous network operation, where one would have toadditionally account for all possible phase-shifts between the twonetworks.

A solution to the aforementioned problems is herein proposed based on adecoupled modulation concept instead. More specifically, each converterside, i.e., three- and single-phase sides of the first and secondnetworks, respectively, are modulated independently. The modulationresults are then mapped into branch level command signals using amapping function and are then fed to a capacitor voltage balancingalgorithm, e.g. a sorting and selection algorithm, for finaldetermination of each cell switching signal (firing signals). Accordingto the chosen modulation method for each side, a choice for circulatingcurrent control implementation may also be used.

The per branch 12 modulation methods are quite attractive due to thefact that they can be implemented in a distributed manner together withthe cell capacitor voltage balancing algorithm. In the direct AC/AC MMC1, this means that both modulation references resulting from therespective sides of the first and second networks L and R are fed intothe same modulator. In case of a voltage signal-based modulation scheme,the global modulation references for the upper and lower branches 12 aand 12 b of the same phase-leg 11 have to be therefore calculated as

$\begin{matrix}{{u_{ka}^{*}(t)} = {\frac{u_{R}^{*}(t)}{2} - {u_{LK}^{*}(t)} - {u_{cm}^{*}(t)} - {u_{circk}^{*}(t)}}} & (1) \\{{u_{kb}^{*}(t)} = {\frac{u_{R}(t)}{2} + {u_{LK}^{*}(t)} + {u_{cm}^{*}(t)} - {u_{circk}^{*}(t)}}} & (2)\end{matrix}$where k=1, 2, 3 correspond to the three different phases, u_(R)*(t),u_(Lk)*(t) denote the result of the first L and second R network sideapplication control, respectively, u_(cm)*(t) is a freely chosencommon-mode voltage component and u_(circk)*(t) is the result of the MMCinner control (branch capacitor voltage unbalance mitigation andcirculating current injection).

However, such an implementation simplicity comes at the cost of aharmonic interaction between the two networks L and R, especially at lowcell numbers and switching frequencies. In order to demonstrate thiseffect, an ideal modulation per branch with three-phase frequency ƒ₃=50Hz and single-phase frequency ƒ₁=50/3 or 16 Hz was simulated withoutconsidering any ripples on the capacitor voltages. The harmonics wereanalysed using the IEC 61000-4-7 standard regarding harmonics andinterharmonics. The Digital Fourier Transform (DFT) was performed over anumber of ten fundamental periods for each side.

Such a modulation per branch does not guarantee quarter-wave symmetry inthe switching pattern at each side, which has an effect on the harmonicspectrum. Therefore it can be said that according to the frequency ratiobetween the two grid frequencies f₃ and f₁, a different harmoniccoupling may be observed. The results can be summarized as follows, with{k,n}∈N,{k,n}>1:

-   -   kf₁=f₃: inter-harmonics appear on the three-phase side;    -   f₁=nf₃: even harmonics appear on the single-phase side and no        common-mode harmonic cancellation occurs on the three-phase        side;    -   f₁=f₃: no inter-harmonics on either side but no common-mode        harmonic cancellation occurs on the three-phase side (two        frequencies coincide);    -   kf₁=nf₃: inter-harmonics appear on both sides.

More sophisticated modulators, such as OPPs, cannot practically beapplied using such an approach per branch 12.

A generalized and simplified block diagram of the proposed modulationconcept is illustrated in FIG. 3. Each network side (three-phase, 3 ph,of the side of the first network L and single-phase, 1 ph, of the sideof the second network R) is modulated 31 independently, having receivedan appropriate reference signal from the upper-layer applicationcontrollers, here denoted in a general manner as Ref_1ph and Ref_3ph,respectively. These could be time-varying voltage or virtual fluxsignals, modulation indexes with respective angle etc. They could alsohave different dimensions according to the chosen method. The modulationresults are integer side-level command signals N_(Rk) for the secondnetwork R and N_(Lk) for the first network L, for each phase leg 11(k=first, second, third phase). These side-level signals are mapped 32to branch-level command signals, which may be within the range of −N to+N where N is the number of cells 13 in each branch 12, in order to befed to the capacitor voltage balancing algorithm. A branch-level commandsignal of +N indicates that all cells in the branch should have thestate +1, while −N indicates that all cells in the branch should havethe state −1 (lower absolute values indicate that one or several cellsin the branch should be bypassed and thus have the state 0). Thesebranch signals are here denoted N_(ka) for the first branch 12 a andN_(kb) for the second branch 12 b. The modulation signals may be updatedat least with the same frequency as the branch equivalent switchingfrequency (i.e. cell output-level equivalent switching frequency timesthe number of cells 13 in each branch 12). By means of the mapping 32 itis determined which combination of cell states, +1, −1 or 0 (bypassed)each of the branches 12 needs to at the same time achieve the first andsecond waveforms of the first and second AC networks L and R,respectively, as previously independently modulated 31. The mappingallows the modulation 31 to be performed on a side level, rather than ona branch level, in accordance with the present invention. Generally, thefirst waveform (of the three-phase first network L) is achieved for theMMC topology of FIG. 1, for each phase leg 11, as half of the differencein combined cell states between the first and second branches 12 a and12 b of that phase leg. The second waveform (of the single-phase secondnetwork R) is at the same time achieved as the sum of the combined cellstates of the first and second branches 12 a and 12 b of the respectivephase leg 11.

Such decoupled modulation was also simulated and it was shown that thedecoupled modulation concept leads to quarter-wave symmetry on bothsides, As a result, the voltage spectrum of both sides is free of anyinterharmonics. Moreover, common-mode harmonics cancel out on the levelof the three-phase line voltage. Furthermore, the decoupled modulationis insensitive to asynchronous operation or the different possiblephase-shifts between the three-phase and single-phase grids L and R.

A further improvement regarding modulation towards the single-phaseconverter side may be achieved by means of interleaving between thethree individual converter phase legs 11, i.e. by switching in each ofthe legs 11 in sequence rather than simultaneously. The increasedvoltage resolution on the single-phase side leads to a lowersingle-phase harmonic spectrum. It is noted that depending on the choseninterleaving method, the signals N_(Rk) of dimension k might besubstituted by the signals N_(Rka) and N_(Rkb) referring to the firstand second (12 a) and (12 b) branches of the phase k. In this case andwithout loss of generality, the mapping function would instead beN _(ka) =N _(Rka) −N _(Lk)  (3)N _(kb) =N _(Rkb) +N _(Lk)  (4)

Three different methods for single-phase side interleaving are possible:

(a) Phase-leg interleaving: This implies that the three phase-legs 11are switched asynchronously, so as to create a larger number of voltagesteps towards the single-phase side of the second network R. This leadsto a maximum of 3(2N)+1 voltage levels in the single-phase (second)waveform.

(b) Common-mode voltage shift interleaving: This implies switching allupper 12 a and all lower 12 b branches simultaneously. This method leadsto a maximum of 2(2N)+1 levels towards the single-phase side. This willlead to a common-mode voltage on the three-phase side of the firstnetwork L. The latter may be distributed to the converter 1 in a mannerso that its zero average value can be guaranteed.

(c) Combined interleaving: This refers to the combination of methods (a)and (b), i.e., the interleaving between the upper and lower branches ofthe three phase-legs as well as interleaving between the threephase-legs. This would lead to a maximum of 6(2N)+1 levels towards thesingle-phase side.

It is noted that according to the chosen modulation method, specialcontrol and logic functions might have to be implemented in order toensure uniform switching frequency among all branches as well as zeroaverage common-mode voltage.

Embodiments of the present invention may be conveniently implemented,e.g. in the controller 14, using one or more conventional generalpurpose or specialized digital computer, computing device, machine, ormicroprocessor, including one or more processors or other processingcircuitry e.g. field-programmable gate array (FPGA), memory and/orcomputer readable storage media programmed according to the teachings ofthe present disclosure. Appropriate software coding can readily beprepared by skilled programmers based on the teachings of the presentdisclosure, as will be apparent to those skilled in the software art.

In some embodiments, the present invention includes a computer programproduct which is a non-transitory storage medium or computer readablemedium (media) having instructions stored thereon/in which can be usedto program a computer to perform any of the methods/processes of thepresent invention. Examples of the storage medium can include, but isnot limited to, any type of disk including floppy disks, optical discs,DVD, CD-ROMs, microdrive, and magneto-optical disks, ROMs, RAMs, EPROMs,EEPROMs, DRAMs, VRAMs, flash memory devices, magnetic or optical cards,nanosystems (including molecular memory ICs), or any type of media ordevice suitable for storing instructions and/or data.

The decoupled modulation in accordance with the present invention workswell under ideal conditions, i.e., when considering zero cell capacitorvoltage ripples. In reality, there may still be some coupling because ofthe existence of capacitor voltage ripples. However, this coupling maybe identified and compensated for by choosing a suitable modulationmethod for each side.

Example 1

FIG. 4 illustrate a more specific example than the general illustrationin FIG. 3, by using well-known modulation methods nearest levelmodulation (NLM) or carrier-based pulse width modulation (CB-PWM). Whensuch a well-known modulation method is used, it is noted that in orderto account for the significant capacitor voltage variations in themodulation process, the modulation references are usually normalizedwith the measured branch capacitor voltage sums, leading therefore tou _(ka) ^(norm)(t)=u _(ka)*(t)/Σu _(ka) ^(cell)(t)  (5)u _(kb) ^(norm)(t)=u _(kb)*(t)/Σu _(kb) ^(cell)(t)  (6)

In order to implement a decoupled modulation scheme for this casewithout losing the information for the circulating current-drivingvoltage u_(circk)* or the branch capacitor voltage sums, the respectivecontrol diagram may have to be modified as shown in FIG. 4.

It is noted that the circulating current control is performed by havingthree individual references for the single-phase side, i.e., perconverter phase leg 11. Moreover, additional logic may be implementedfor the single-phase side modulation, in order to achieve interleavingtowards the side of the second network R. It is also noted that the sixbranch 12 capacitor voltage sums are not the only option for normalizingthe branch modulation references. One could choose any normalizationfactor according to a desired performance.

Example 2

In this example, a more advanced modulator scheme is used. Asillustrated in FIG. 5, there is no need for compensating for the voltageripples by normalizing with the measured branch capacitor voltage sums,since the latter is performed on the level of the modulation algorithminstead.

Therefore, the diagram may be redrawn as shown in FIG. 5. It is notedthat in such a case, additional feedback signals are needed on the levelof the modulation algorithms. These include measurements, e.g., branchcurrents, as well as parameters, e.g., value of the cell capacitance.

Example 3

In this example, illustrated in FIG. 6, closed-loop controlled OPPs areused, e.g. MP3C. In the lower power range of the example case of railwayapplications, only a few number of cells may be used. In addition, thesingle-phase voltage of the second network R is given by the catenary,therefore the three-phase voltage side of the first network L is usuallyscaled according to the nominal semiconductor current. This could leadto the under-utilization of the number of voltage levels towards thethree-phase side.

The use of OPPs implies an active shaping of the harmonic spectrum. Insuch a case, the concept of decoupled modulation offers the capabilityof treating the two converter sides independently and therefore nothaving to design and apply the OPP directly on a branch level. Thelatter would have been nearly impossible to achieve given the existenceof two system frequencies but also an infinite number of phase-shiftsbetween the two networks L and R in the asynchronous grid operationcase.

However, OPPs cannot be easily combined with a closed-loop controlsystem for several reasons. First, the discontinuity of the switchingangles when varying the modulation index prevents the use of linearcontrollers due to stability issues. Second, unlike modulation methodswith a fixed modulation cycle such as CB-PWM and space vector modulation(SVM), OPPs are associated with a non-zero ripple current at the end ofthe switching period. The bandwidth of a conventional current controllerthus has to be slow, limiting the capability of the controller to rejectdisturbances and to react to transients. Last but not least, OPPs aretypically calculated considering ‘ideal’ conditions, i.e., neglectingsystem disturbances. This limits the applicability of OPPs to MMC 1configurations without additional necessary control actions, due to theconsiderable inherent capacitor voltage ripples and other systemnon-idealities.

Due to above stated reasons, an advanced closed-loop control andmodulation method, such as Model Predictive Pulse Pattern Control(MP3C), may be utilized in conjunction with the offline precomputedOPPs. MP3C controls the (virtual) flux vector along a (virtual) fluxreference trajectory. The latter is obtained by integrating up thenominal switched voltage waveform of the OPP. The difference between the(measured or estimated) flux vector and its reference is the flux error.By modifying the switching instants of the OPP over a predictionhorizon, the flux error may be minimized and closed-loop control of the(virtual) flux and thus of the currents may be achieved.

Only the first part of the modified OPP within the time interval [t,t+T_(s)], where T_(s) is the sampling interval, is applied to theconverter 1. At the next sampling instant, using new measurements (orestimates), the flux error minimization is repeated over a shifted orreceding prediction horizon. This so-called receding horizon policyprovides feedback and ensures that the controller is robust to parameteruncertainties. The condition on the horizon is to have at least twoswitching events in two different phases in order to be able to drivethe flux error to zero.

Originally, the concept of MP3C has been developed for three-phasesystems. As a straightforward solution, the MP3C concept is utilizedonly on the three-phase side of the first network L. The single-phaseside of the second network R may therefore be modulated simply usinge.g. PSC-PWM, leaving the possibility of performing circulating currentcontrol from the single-phase side. This is depicted in the simplifiedblock diagram of FIG. 6. It is noted that since MP3C is a three-phasecontrol model, the converter flux reference in stationary referenceframe ψ_(c,αβ)* has to be provided to the controller 14. The latter isbased on the chosen OPP for the specific operating point.

Alternatively, OPPs may be also designed and used for the single-phaseside of the second network R as well. To this end, the MP3C concept maybe modified to the single-phase side, by considering only one phase andtracking the flux reference in this given phase.

A modified block diagram is shown in FIG. 7. The three-phase side of thefirst network L is now responsible for controlling the circulatingcurrent by employing a dead-beat circulating current controller (DBC3).The single-phase side of the second network R is operating with a scalarMP3C. The interleaving block needs information regarding the state ofboth sides, therefore it is drawn outside of the single-phase modulationblock.

The present disclosure has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the present disclosure, as definedby the appended claims.

The invention claimed is:
 1. A method of decoupled modulation of adirect AC/AC modular multilevel converter (MMC) between a first ACnetwork having a first waveform and a second AC network having a secondwaveform, the MMC having a double-star topology with a plurality ofphase legs, each phase leg having a first branch and a second branch,each of the first and second branches comprising a plurality of seriesconnected bipolar cells, the method comprising: performing a firstmodulation based on a reference signal of the first AC network,independently of a reference signal of the second AC network, togenerate, for each phase leg, a first integer command signalcorresponding to a first combination of cell states in the first andsecond branches of the phase leg needed for generating the firstwaveform; performing a second modulation based on the reference signalof the second AC network, independently of the reference signal of thefirst AC network, to generate, for each phase leg, a second integercommand signal corresponding to a second combination of cell states inthe first and second branches of the phase leg needed for generating thesecond waveform; based on the first and second integer command signals,mapping to each branch a number of cell states to be used forconcurrently generating both the first and second waveforms, generatingbranch-level command signals to a capacitor voltage balancing algorithm;and based on the mapping and the balancing algorithm, sending firingsignals to the plurality of cells of each branch; wherein the first ACnetwork is a three-phase AC network; wherein the second AC network is asingle-phase AC network.
 2. The method of claim 1, wherein the second ACnetwork is a railway electrification network.
 3. The method of claim 1,wherein the first AC network has a nominal frequency of 50 Hz or 60 Hz.4. The method of claim 1, wherein the second AC network has a nominalfrequency of 25 Hz, 50/3 Hz or 16.70 Hz.
 5. The method of claim 1,wherein the plurality of cells of each branch is less than 20 cells. 6.The method of claim 1, wherein each of the plurality of cells has aswitching frequency of at most 150 Hz.
 7. The method of claim 1, whereinthe MMC has an operating voltage of less than 30 kV.
 8. The method ofclaim 1, wherein the MMC has an operating power within the range of 10to 60 MW.
 9. A computer program product comprising computer-executablecomponents for causing a controller of the MMC to perform the method ofclaim 1 when the computer-executable components are run on processorcircuitry comprised in the controller.
 10. A direct AC/AC modularmultilevel converter (MMC) configured to be connected between a first ACnetwork being a three-phase AC network and having a first waveform and asecond AC network being a single-phase AC network and having a secondwaveform, the MMC having a double-star topology with a plurality ofphase legs, each phase leg having a first branch and a second branch,each of the first and second branches comprising a plurality of seriesconnected bipolar cells, the MMC further comprising a controllercomprising: processing circuitry; and storage storing instructionsexecutable by said processor circuitry whereby said controller isoperative to: perform a first modulation based on a reference signal ofthe first AC network, independently of a reference signal of the secondAC network, to generate, for each phase leg, a first integer commandsignal corresponding to a first combination of cell states in the firstand second branches of the phase leg needed for generating the firstwaveform; perform a second modulation based on the reference signal ofthe second AC network, independently of the reference signal of thefirst AC network, to generate, for each phase leg, a second integercommand signal corresponding to a second combination of cell states inthe first and second branches of the phase leg needed for generating thesecond waveform; based on the first and second integer command signals,map to each branch a number of cell states to be used for concurrentlygenerating both the first and second waveforms, generating branch-levelcommand signals to a capacitor voltage balancing algorithm; and based onthe mapping and the balancing algorithm, send firing signals to theplurality of cells of each branch.
 11. The MMC of claim 10, wherein thesecond AC network is a railway electrification network.
 12. The MMC ofclaim 10, wherein the first AC network has a nominal frequency of 50 Hzor 60 Hz.
 13. The MMC of claim 10, wherein the second AC network has anominal frequency of 25 Hz, 50/3 Hz or 16.70 Hz.
 14. The MMC of claim10, wherein the plurality of cells of each branch is less than 20 cells.15. The MMC of claim 10, wherein each of the plurality of cells has aswitching frequency of at most 150 Hz.
 16. The MMC of claim 10, whereinthe MMC has an operating voltage of less than 30 kV.
 17. The MMC ofclaim 10, wherein the MMC has an operating power within the range of 10to 60 MW.
 18. The method of claim 2, wherein the first AC network has anominal frequency of 50 Hz or 60 Hz.
 19. The method of claim 2, whereinthe second AC network has a nominal frequency of 25 Hz, 50/3 Hz or 16.70Hz.
 20. The method of claim 3, wherein the second AC network has anominal frequency of 25 Hz, 50/3 Hz or 16.70 Hz.